1. Field of the Invention
The invention relates to the field of electronics. More specifically, the invention relates to high frequency electromagnetic interference (xe2x80x9cEMIxe2x80x9d).
2. Background of the Invention
An inverter is commonly used to convert direct current (xe2x80x9cDCxe2x80x9d) to alternating current (xe2x80x9cACxe2x80x9d) to power a three-phase load, such as a three-phase motor, or, alternatively, to convert AC from a three-phase source to DC. The inverter commonly contains six switches. Power modules often contain one or more pairs of complementary switches. The power module typically includes silicon dice on substrates that are secured to the module baseplate. Each switching pair has a positive or xe2x80x9chighxe2x80x9d side switch and a negative or xe2x80x9clowxe2x80x9d side switch for controlling the flow of electric current. Each switching pair is referred to herein as a xe2x80x9chalf bridge.xe2x80x9d The xe2x80x9chigh sidexe2x80x9d of the bridge contains the positive switches, and the xe2x80x9clow sidexe2x80x9d contains the negative switches. By the term xe2x80x9cswitchxe2x80x9d is meant a switching device such as an insulated gate bipolar transistor (xe2x80x9cIGBTxe2x80x9d) or Bipolar Junction Transistor (xe2x80x9cBJTxe2x80x9d) or Metal Oxide Semiconductor Field Effect Transistor (xe2x80x9cMOSFETxe2x80x9d), either singly or in parallel.
Elements may be described herein as xe2x80x9cpositivexe2x80x9d or xe2x80x9cnegative.xe2x80x9d An element described as xe2x80x9cpositivexe2x80x9d is shaped and positioned to be at a higher relative voltage than elements described as xe2x80x9cnegativexe2x80x9d when the power module is connected to a power source. xe2x80x9cPositivexe2x80x9d elements are positioned to have an electrical connection that is connectable to the positive terminal of a power source, while xe2x80x9cnegativexe2x80x9d elements are positioned to have an electrical connection that is connectable to a negative terminal, or ground, of the power source. Generally, xe2x80x9cpositivexe2x80x9d elements are located or connected to the high side of the power module and xe2x80x9cnegativexe2x80x9d elements are located or connected to the low side of the power module.
In a typical power module configuration, the high side switches are on one side of the module opposite the corresponding low side switches. A positive DC lead from a power source such as a battery is connected to a conducting layer in the high side of the substrate. Likewise, a negative DC lead from the power source is connected to a conducting layer in the low side of the substrate. The high side switches control the flow of current from the conducting layers of each high side substrate to output leads. Output leads, called xe2x80x9cphase terminalsxe2x80x9d transfer alternating current from the three pairs of switches, or half bridges, to the motor.
Power modules typically have three half bridges combined into a single three-phase switching module, or single half-bridge modules that may be linked together to form a three-phase inverter. As would be understood by one of ordinary skill in the art, the same DC to AC conversion may be accomplished using any number of half bridges, which correspond to a phase, and each switching pair may contain any number of switching devices. For simplicity and clarity, all examples herein use a common three phase/three switching pair configuration. However, the invention disclosed herein may be applied to a power module having any number of switches.
Current flows from the power source through the positive DC lead to the conducting layer on the high side substrate. Current is then permitted to flow through one or more switching device on the high side to a conducting layer, commonly referred to as a phase output layer, on the low side. A phase terminal lead allows current to flow from this conducting layer on the low side to the motor. The current then flows from the motor to the corresponding conducting layer on the low side of a second switching pair through the low side switches and diodes to the negative DC lead to the power source.
Current flowing through various inductive paths within the module transiently stores energy which increases energy loss, reduces efficiency, and generates heat. When the flow of current changes, as in such a high frequency switching environment, large voltage overshoots often result, further decreasing efficiency. Additional materials regarding efficient configurations of power modules may be found in application Ser. No. 09/957,568, entitled xe2x80x9cSubstrate-Level DC Bus Design to Reduce Module Inductance,xe2x80x9d application Ser. No. 09/957,047, entitled xe2x80x9cPress (Non-soldered) Contacts for High Current Electrical Connections in Power Modules,xe2x80x9d and application Ser. No. 09/882,708, entitled xe2x80x9cLeadframe-Based Module DC Bus Design to Reduce Module Inductance,xe2x80x9d which are hereby incorporated by reference in their entirety.
To minimize the negative effects of current gradients, noise and voltage overshoots associated with the switching process of the module, large capacitors are generally placed in a parallel arrangement between the positive and negative DC connections or from each DC connection to a ground or chassis. These large capacitors are commonly referred to as xe2x80x9cXxe2x80x9d or xe2x80x9cYxe2x80x9d capacitors. Relatively large external capacitors of about around 100 micro Farads are needed. By xe2x80x9cexternalxe2x80x9d it is meant that the element referred to is located outside of a power module. High frequency noise, and voltage overshoots that are initiated in the module by the switching process travel away from the source of the noise and voltage overshoots. A low impedance network may be used to provide a return path for the high frequency energy associated with noise and voltage overshoots. The further the energy travels, the more difficult it is to provide a low impedance network to return the energy. Therefore, capacitors attached between the positive and negative DC connections or from the DC connections to ground must be relatively large to minimize the impact of noise, and voltage overshoots. In addition, these external capacitors typically cause stray inductance, which renders the capacitor ineffective at frequencies higher than about 10 kHz.
These and other problems are avoided and numerous advantages are provided by the method and device described herein.
The present invention provides high frequency, low impedance network for use in a power module for reducing radiated and conducted electromagnetic interference and the resulting noise and voltage overshoots. By xe2x80x9ca high frequency, low impedance networkxe2x80x9d it is meant any structure characterized by an equivalent impedance below about 10 nanoHenry (xe2x80x9cnHxe2x80x9d), and typically between about 100 picoHenry and about 10 nH, in a frequency range from between about 10 Mega Hertz (xe2x80x9cMHzxe2x80x9d) to about 1 Giga Hertz (xe2x80x9cGHzxe2x80x9d). Because the high frequency, low impedance is located relatively close to the source of noise and voltage overshoots inherent in the switching process, a much smaller capacitance may be used with more effective reduction of noise and voltage overshoots when compared to larger, external capacitors.
Elements may be described herein as xe2x80x9cadjacentxe2x80x9d to another element. By the term xe2x80x9cadjacentxe2x80x9d is meant that in a relationship so characterized, the components are located proximate to one another, but not necessarily in contact with each other. Normally there will be an absence of other components positioned in between adjacent components, but this is not a requirement. By the term xe2x80x9csubstantiallyxe2x80x9d is meant that the orientation is as described, with allowances for variations that do not effect the cooperation and relationship of the so described component or components.
In accordance with the present invention, a method for reducing electromagnetic interference in a power module is provided. A high frequency, low impedance network is electrically connected to at least one of a positive conducting layer in a substrate or a negative conducting layer in a substrate. The high frequency, low impedance network is also electrically connected to ground.
In another aspect, a device is provided for reducing electromagnetic interference in a power module. The device includes a surface mount capacitor, a first electrical connection from the surface mount capacitor to at least one of a positive conducting layer in a high side substrate of a power module or a negative conducting layer in a low side substrate of a power module, and a second electrical connection from the surface mount capacitor to ground.
In one aspect, the first electrical connection is a soldered connection.
In another aspect, the second electrical connection includes a via connection from the surface mount capacitor to an electrically grounded layer in the substrate of a power module. In still another aspect, the second electrical connection includes an electrically isolated substrate layer soldered to the surface mount capacitor and a wire bond from the electrically isolated layer to a ground connection in a power module. Preferably, the surface mount capacitor is between about 1 and about 100 nano Ferads.
In yet another aspect of the invention, a power module for reducing inductance is disclosed. The module has a lead frame for supporting the module and for providing interconnections to the motor and the power source. A substrate, which includes a high side substrate and a low side substrate, is connected to the lead frame. High side switches are proximate to the high side substrate and low side switches are proximate to the low side substrate. A positive conducting layer in the high side substrate is configured for connection to a positive bus and a negative conducting layer in the low side substrate is configured for connection to a negative bus. A capacitor is electrically connected to at least one of the positive conducting layer or the negative conducting layer, and a ground is electrically connected to the substrate.
According to the invention, the method, device and power module disclosed herein provide improved efficiency and more even motor performance through the reduction of electromagnetic interference in a power module. Because the capacitor is located in the substrate of the power module, a smaller and less expensive capacitor arrangement may be used to reduce electromagnetic inductance.
These and other advantages will become apparent to those of ordinary skill in the art with reference to the detailed description and drawings.